Seat belt retractors are a standard component of motor vehicle belt restraint systems and have a spool (spindle) for receiving seat belt webbing. The spool is used to wind up and store the webbing. The spool is locked against rotation upon detection at a potential accident situation in order to restrain the occupant via the seat belt. Recently, retractors have been designed having one or more load limiting elements which are structured to allow the spool to rotate and pay out the seat belt webbing upon reaching predetermined webbing load levels between the occupant and seat belt during a restraint event. In this manner, the restraint force imposed on the occupant can be limited in a controlled manner, providing desired load limitation characteristics.
More recent enhancements in load limiters have been designed to provide multilevel load limiting capabilities. For example, higher restraining forces may be initially applied, followed by lower restraining forces at a later point during an emergency event, or a low to high load profile can be provided. Torsion bars arranged coaxially within the spool are commonly used as load limiting elements. In an impact condition, one end of the torsion bar is locked to the retractor frame while the other end is coupled with the retractor spool. The bar section between the attachment points undergoes elastic and plastic torsional deflection, enabling torsion controlled relative rotation between the spool and the retractor frame. The resulting controlled extraction of webbing during a restraint event serves to limit belt loading acting on the vehicle occupant.
One type of multi-stage load limiting retractor uses a multi-stage torsion bar or a system of torsion bars. The multi-stage torsion bar is essentially two torsion bars that are axially aligned and joined at respective ends. The appropriate stage or portion of the torsion bar may be selectively coupled to provide a secondary load limiting characteristic as desired.
Presently available torsion bar type load limiting retractors generally operate satisfactorily. The more sophisticated multilevel load limiting systems also operate in an intended manner. However, there are additional design goals and objectives desired for further improvement. One such design goal is providing a mechanism for limiting the total rotational deflection provided by a torsion bar load limiting element, enabling multiple turns of relative rotation. In addition, in an effort to accurately tailor load limiting characteristics to design criteria, sophisticated digressive and progressive load limiting profile systems have been developed. These systems enable the force load profile of the retractor to be tailored to increase (progressive) or decrease (digressive) over extraction of the webbing. Although retractors having such capabilities are known, the related systems become complex, costly, and can impose packaging size disadvantages. Further design goals include the ability to easily adapt a retractor to provide specific load limiting characteristics to a particular vehicle application along with preferably a low part count, and low cost.